The invention relates generally to marine propulsion devices, and more specifically to marine propulsion devices having a thrust washer on the propeller shaft.
The use of thrust washers in marine propulsion devices to position a propeller on a propeller shaft and to transmit thrust from the propeller to the propeller shaft is known. In general, prior art thrust washers are machined with a frustoconical or tapered inner surface and are designed to be placed over the propeller shaft to engage a portion of the propeller shaft having a frustoconical or tapered outer surface. When the thrust washer is placed on the shaft, the tapered surface of the thrust washer engages the tapered surface of the propeller shaft. The engaged tapered surfaces form a contact zone. Both thrust loads developed by rotation of the propeller and any bending loads caused by hydrodynamic effects on the propeller are transferred from the propeller to the propeller shaft through the contact zone.
Prior art thrust washer designs present difficulty when the tapered surfaces of the propeller shaft and the thrust washer do not match. Poor manufacture or assembly of the thrust washer or the propeller shaft can result in mismatched tapered surfaces. Mismatched thrust washers and propeller shafts can result in a highly loaded contact zone which, in the worst case, is a line contact between the thrust washer and the shaft around the tapered portion of the propeller shaft. The smaller the contact zone, the greater the concentration of stress, and the greater the risk of failure due to fatigue. Accordingly, prior art designs require the expensive machining of the frustoconical mating surfaces on the thrust washer and the propeller shaft to narrow tolerance dimensions. Also, additional measures may have to be taken to assure that an assembled thrust washer and propeller shaft are properly matched.
Prior art thrust washer designs present additional difficulty, because the thrust washer acts on a tapered surface. Loads from the propeller are concentrated by the thrust washer on the reduced diameter portion of the shaft instead of on the larger diameter of the shaft. Bending loads on a reduced diameter also concentrate stress.
Attention is directed to the thrust washers disclosed in the following U.S. Patents: U.S. Pat. No. 4,793,773 issued to Kinouchi, et al on Dec. 27, 1988; U.S. Pat. No. 4,566,855 issued to Costabile, et al on Jan. 28, 1986; U.S. Pat. No. 4,545,771 issued to Iio on Oct. 8, 1985; U.S. Pat. No. 3,876,332 issued to Kashmerick on Apr. 8, 1975; U.S. Pat. No. 3,759,076 issued to Reese on Sep. 18, 1973; U.S. Pat. No. 3,444,932 issued to Wlezien on May 20, 1969; U.S. Pat. No. 3,389,558 issued to Hall on Jun. 25, 1968; and U.S. Pat. No. 3,335,803 issued to Van Ranst on Aug. 15, 1967.